1. Field of the Invention
The present invention relates to a thermal transfer type recording apparatus and method, capable of recording an image in a continuous gradation.
2. Description of the Related Art
Along with the recent advanced technology of recording in colors data from the video camera, computer graphic data, etc., the demands for recording in a single color as well as for hard copying in colors have become greater. To accommodate such demands, color hard copying techniques such as the sublimation type thermal printing, fusion type thermal printing, ink-jet printing, electrophotographic printing, heat development type silver film image printing, etc. have been proposed. Among these printing techniques, the dye diffusion thermal printing technique (sublimation type thermal printing) and ink-jet printing technique can be implemented by a simple apparatus to easily provide quality images.
Of these recording techniques, the dye fusion thermal printing technique is such that an ink ribbon or ink sheet having coated thereon an ink layer of a suitable binder resin in which a transferrable dye of a high concentration is dispersed, and a print receptor coated thereon with a dyeing resin to accept the transferred dye, such as a printing paper, are put into close contact with each other under a predetermined pressure, a heat corresponding to image information is imparted to the ink sheet from a thermo-sensitive printer head on the ink sheet, and thus the transferred dye is thermally printed from the ink sheet to the print receptor correspondingly to the heat quantity impaired from the dyeing resin layer.
The above operations are repeated for each of image signals decomposed into three subtractive primaries: yellow, magenta and cyan, thereby providing a full color image having a continuous gradation. This is a so-called dye decomposition thermal transfer type printing technique which permits to implement a recording apparatus designed compact, easy to maintain, ready to start up for printing and capable of providing an image having a quality as high as that assured by the silver film image color photography.
However, this technique is not advantageous in that the disposable ink sheet will result in a big waste and the running cost is large. Thus, the technique has not being prevailing. This is also true with the fusion type thermal transfer type printing technique.
The conventional thermal transfer type printing provides quality images but the dedicated printing paper and disposable ink ribbon or ink sheet lead to a high running cost.
On the other hand, the ink-jet printing technique is such that as disclosed in the Japanese Unexamined Patent Publication Nos. 86-59911 and 93-217, a small droplets of printing ink are jetted from a nozzle provided in a printer head to the surface of a printing material by one of the electrostatic attractive force technique, continuous vibration (piezo) technique, thermal printing (bubble-jet) technique, etc. selected correspondingly to image information to be printed and the printing ink adheres to the printing material, thereby forming an image on the printing material.
Therefore, printing can be made on a plain paper and the running cost is low with little waste resulted from the disposable ink ribbon. Recently, since the thermal (bubble-jet) recording method permits to easily provide a color image, so it is prevailing.
Because of the principle, however, the ink-jet printing technique can hardly print an image with an intra-pixel density gradation, and thus reproduce in a short time a quality image having a quality as high as that of a silver film image obtainable using the dye diffusion type thermal printing method.
That is, since one droplet of ink forms one pixel, the conventional ink-jet printing technique is hard to print an image with an intra-pixel gradation and cannot form an image having a high quality. It has been tried to utilize the high resolution of the ink-jet printing for representation of a pseudo gradation by the dither method. However, this approach cannot assure the same image quality as that in the sublimation thermal printing and the printing speed is considerably lower than that in the sublimation thermal printing.
To solve the above-mentioned problems encountered with in the field of the prior art, it has been proposed to use a so-called mist of ink in the ink-jet printing in order to micronize the ink droplet to be jetted. This technique generally includes two methods: one is the ultrasonic vibration method and the other is the satellite droplet method. In the ultrasonic vibration method, an ultrasonic vibration is generated at an ink nozzle using mainly a piezo transducer to develop a surface tension vibration under which the ink droplets collide with each other to produce an ink mist which is to be jetted towards, and thus to adhere to, the surface of a printing material. On the other hand, the satellite droplet method is such that micro droplets of ink generated derivatively just after generation of main droplets of ink are utilized to form an image on the printing material but the main droplets of ink are not used for the image forming.
Generally, it is difficult to microstructure the piezo element. So, the ultrasonic vibration method is disadvantageous in that no line printer head can be built and thus no high printing speed can be attained. Also it is difficult to localize the ultrasonic vibration, and thus the microstructuring of the piezo element is further difficult and the cross-talk is a large problem in the ultrasonic vibration method. To prevent the main droplets of ink from being transferred to the printing paper, the satellite droplet method needs, for example, a suitable means for charging the droplet and deflecting it in an electric field. This method is actually limited to the so-called continuous ink-jet printing technique and so cannot be implemented with a low cost.
To solve the above problems, an ink mist printing technique has recently been proposed.
More specifically, the ink mist printing method utilizes the collision of ink droplets due to a surface tension convection developed as a heater is energized and deenergized, to atomize an ink for transfer to thereby form a quality image. In this method, the well-known semiconductor machining technique can be used to easily dispose heaters highly densely and an appropriate structure can be placed on the heaters to avoid the problems such as cross-talk, etc.
In this method, however, when the printing liquid or ink is heated by the heaters, the ink boiling causes coarse droplets and micro droplets derivatively produced along with main droplets, so-called satellite droplets, which will cause the printing quality to be lower. Namely, it is difficult to stably jet an ink mist by the surface tension convection.